1. Field of the Invention
The present invention relates to a power steering apparatus for a small vessel and more particularly to a power steering apparatus with an electrically driven assist device.
2. Description of the Related Art
For use in a small sized vessel such as a boat provided with a power propulsion unit such as an outboard engine or the like, there is publicly known the art that a power steering apparatus is employed in a steering system. As an example of this known art, a steering system comprises a handle being pivotally supported on an instrument panel on which a meter and the like for a cabin are mounted, a power steering apparatus and an oil hydraulic pump device being mounted in series with each other on a steering shaft of the handle, in which the hydraulic pump device is driven by an assist output power to generate oil pressure such that an outboard engine is rotated by this oil pressure for steering the boat (see a patent reference 1).
This power steering apparatus is an electrically driven assist device which comprises a worm gear being operated by an electric motor, and a worm wheel being engaged with the worm gear to rotate the steering shaft. In the electrically driven assist device, a torque sensor detects a steering torque manually applied to the handle. Based on the detected values, the electric motor is controlled to exert a proper assist torque. The oil hydraulic pump device is a helm pump which is comprised of a swash plate type axial piston pump and which is configured to generate the oil pressure in proportion to the output power of the electrically driven assist device.
Moreover, there is known the art that a handle is provided with a tilt mechanism which makes it possible to adjust a tilt angle relative to an instrument panel and that an oil hydraulic pump device is carried on the instrument panel (see a patent reference 2).
Patent reference 1: Japanese patent laid-open publication No. JP 2005-231383A.
Patent reference 2: Japanese patent laid-open publication No. JP 2000-43794A.
By the way, the above electrically driven assist device is integrally mounted in series with the oil hydraulic pump device with respect to the steering shaft of the handle. Therefore, the power steering apparatus extends long under the instrument panel so that the distance between a bottom of the boat and the instrument panel increases to thereby position the center of gravity of an operator higher. However, it is desirable that the boat which pitches and rolls by catching the waves has the lower center of gravity to stabilize a hull. This requires the length of the power steering apparatus to be shortened and the space of the arrangement of the apparatus under the instrument panel to be decreased, in such a way as to lower the positions of the handle and the operator's seat, so that the center of gravity is lowered.
Further, in the above mentioned power steering apparatus, since the electrically driven assist device and the oil hydraulic pump device are integrally connected in series with each other, the driving force of the electrically driven assist device is transmitted at a ratio of one to one to the oil hydraulic pump device. However, in order to steer clear of complicated waves, it is desirable to provide the steering performance of good response, namely, the effective steering function. For that reason, it is required to enhance the pumping efficiency of the oil hydraulic pump device thereby to improve the steering response.
Furthermore, the torque sensor in the above mentioned patent reference 1 comprises a torque ring adapted to move in an axial direction in response to the steering torque along an input shaft which is connected to the steering shaft, a torque pin projecting from the torque ring, and a detecting element of the torque sensor on which the torque pin slides. When the torque pin slides on the detecting element of the torque sensor, the steering torque is detected from the position of the torque pin. Thus, since the torque pin keeps in direct contact with the detecting element of the torque sensor, the impact against the steering shaft is easily transmitted to the detecting element of the torque sensor. The torque sensor, however, is a precision device. Therefore, when an impact load is imposed on the detecting element of the torque sensor, an error of the detected value increases so as to make it difficult to perform the accurate assist. Moreover, the torque sensor for use in the small vessel such as the boat is in such an environment as to easily take a large impact by the waves or the like. Accordingly, it is desirable to improve the detecting accuracy of the torque sensor even in such environment.
It is, therefore, an object of the present invention to provide the art for shortening the length of a power steering apparatus. Herein, the length of the power steering apparatus means the length in the direction of a rotation shaft of an electrically driven assist device.
Another object of the present invention is to provide a power steering apparatus capable of enhancing the pumping efficiency of an oil hydraulic pump device so as to improve the steering response.
A further object of the present invention is to provide a power steering apparatus capable of improving the detecting accuracy of a torque sensor.